Recruitment of diabetic patients with neovascular retinopathy into the Collaborative Diabetic Retinopathy Study will begin during October l972. Patients selected will be those with neovascularization of comparable degree in both eyes so that no treatment in one eye may be compared with randomly selected treatment in the other eye using Xenon arc and/or Argon Laser photocoagulation. Recruitment of patients should be at the rate of one per one to two weeks for the first seven to eight months. Thereafter assuming a full-time ophthalmologist is available, patients will be entered more rapidly into the study. Prior to recruitment stereo-fundus photographs will be obtained according to the specifications of the Collaborative Study and will be sent to the reading center for assurance of adequate quality. In addition, the reading skill of the Co-Principal Investigator, the optometrist, and a trained technician will be tested for compliance with the standards supplied by the reading center. Light treatment to the eye designated by the Coordinating Center will be carried out by a research fellow in ophthalmology under the supervision of the Co-Principal Investigator.